Resolve "Broken master `cache gems: needs 'setup-test-env pg10'`"

What does this MR do?

setup-test-env pg10 only runs for master with code/backstage changes:

• https://gitlab.com/gitlab-org/gitlab/-/blob/090f4d1fc0f78f37be1c1051b97e46385adf363a/.gitlab/ci/rails.gitlab-ci.yml#L37
• https://gitlab.com/gitlab-org/gitlab/-/blob/090f4d1fc0f78f37be1c1051b97e46385adf363a/.gitlab/ci/rules.gitlab-ci.yml#L359-364

But we now test against PG11 by default, so cache gems should rely on setup-test-env pg11 since it runs for QA changes as well: https://gitlab.com/gitlab-org/gitlab/-/blob/090f4d1fc0f78f37be1c1051b97e46385adf363a/.gitlab/ci/rules.gitlab-ci.yml#L533

Closes #214687 (closed).